Disulfide bond formation is an important post-translational modification, with implications for structure, function and stability. However, in the wrong context disulfides can be detrimental. Evidence has accumulated supporting the important role played by post-translationally modified neoantigens in the pathogenesis of autoimmune diseases. Autoimmune T cell responses that are specific to neoepitopes formed in peripheral tissues may explain how and why T cell responses are not subject to the usual tolerance mechanisms. Recent data showed that posttranslational modifications (PTMs) at the peptides from autoantigens create the pathogenic neoepitopes and may drive T1D. IAPP is a peptide co-secreted with insulin by the beta cells of the pancreatic islets of Langerhans. The first 20 amino acids of the IAPP, KS20, is the target antigen (Ag) for a highly diabetogenic CD4 T cell clone BDC-5.2.9. We determined the crystal structure of KS20 bound to NOD mouse MHC II, I-Ag7. Surprisingly, the KS20 contains an N- terminal disulfide bond between residue 2 and residue 7. This explained why BDC-5.2.9 T cell clone optimally recognizes the KS20 epitope comprising 20 amino acids. This is the first structure of a disulfide modified peptide presented by MHC. Peptides bound to MHCII take a nearly identical course through the binding groove stabilized by a set of conserved hydrogen bonds between MHCII amino acids lining the binding groove and the peptide backbone. The disulfide bond of the KS20 Ag bulged out of the I-Ag7 peptide binding groove and formed a large loop on top of I-Ag7. This uniquely modified antigen implies redox may play an important role in Ag processing and presentation. Our goal is to characterize the disparate T cell recognition of the disulfide modified peptides, and understand the novel Ag processing and presentation mechanisms used to generate the unconventional T cell responses to this ligand. We hypothesize that the KS20 reactive CD4 T cells may adopt an unconventional binding mode when recognizing the I-Ag7/KS20 complex. Increasing evidence shows that reactive oxygen species (ROS) induces post-translational modifications. ROS can also readily oxidize the free thiol group. Gamma- interferon inducible lysosomal thiol reductase (GILT) participates in Ag processing by catalyzing the reduction of disulfide bonds, thus facilitating the unfolding of native S-S bond containing Ag for further processing. We hypothesize that ROS may impact CD4 T cell Ag presentation and GILT redox regulation may affect IAPP Ag processing. We propose to test how disulfides activate CD4 T cells and investigate redox regulation of disulfide modified Ags.